This invention relates to the field of spectrometry, and to that type of sample analysis in which an internal reflection element is immersed in a sample, usually a liquid material.
As discussed in a copending application having the same inventor and, the same assignee as this application (Ser. No. 158,214 filed 2/19/88), an accessory referred to as a "prism liquid cell" is marketed by Harrick Scientific Corporation. This accessory is used to replace "amalgamated sealed cells," which were previously used to hold sample material, through which previously used to hold sample material, through which radiation was passed to provide transmission spectroscopy.
In internal reflection spectroscopy by an accessory extending into the sample material, an internally reflecting element (IRE) is surrounded by the sample; and the analytical radiation is essentially confined inside the IRE. Infrared light may enter and leave the IRE from the same end, which may have the shape of a 90.degree. rooftop. The Harrick accessory is a rectangular cross-section crystal (typically composed of zinc selenide) having its outer (non-immersed) end cut at an angle of 45.degree. to each of its long sides.
Infrared light from a source enters the first inclined side of the rooftop, generally on a perpendicular path. The infrared light is reflected first from one wall of the IRE, then from the opposite wall of the IRE, and so on, until it reaches the end of the IRE. It is then returned, along a path parallel to the incoming radiation, by reflection back and forth across the IRE until it exits the IRE from the second inclined side of the rooftop. And it is then directed to an infrared detector.
If the IRE (also referred to as a prism or crystal) is surrounded by air, the internal radiation from the source will be totally reflected. However, if the IRE is in contact with an infrared absorbing material, such as a liquid chemical, the radiation will be selectively absorbed at various wavelengths, resulting in an infrared spectrum. The amount of radiation absorbed is influenced by the angle of the incidence of the radiation on the sides of the IRE.
IREs are often referred to as attenuated total reflectance (ATR) crystals, because the internal reflectance permits a limited amount of light absorption by the sample surrounding the crystal. The angle of incidence of the internal light on the crystal walls must be at or above the critical angle. If the angle of incidence is too small, excessive radiation will leave the crystal and be absorbed by the sample. If the angle of incidence is too large, excessive radiation will be internally reflected, and insufficient radiation will be absorbed by the sample to provide adequate analytical information. In other words, sample absorption of radiation is necessary, but sufficient radiation must return and exit from the crystal on its way to the detector.
A thrust of certain applications assigned to the assignee of the present application is the development of spectrometer systems, and components thereof, which are readily adaptable to different uses, and to uses with different samples. For example, application Ser. No. 900,730, filed Aug. 27, 1986, provides a spectrometer system using components which are both modular and switchable. The switchability provides the versatility of having a number of interconnected units which give the convenience of ready access, while the modularity permits easy and time-saving re-structuring of the accessory combination included in the system.